It was thought that the much shorter fossil record for echidnas, from about 13 million years ago, was just due to the patchy nature of the fossil record," Phillips said. "Platypus-like fossils are known from at least 61 million years ago. A number of aspects of echidna biology are consistent with an amphibious platypus-like ancestor - a streamlined body, rearward-jutting hind limbs that could act as rudders, and the contours of a duck-like bill during embryonic development. This means echidnas recently had semi-aquatic predecessors and only later recolonized the land. However, new genetic evidence and comparisons with fossil monotremes suggests that echidnas only diverged from platypuses 19 to 48 million years ago. Platypuses are amphibious creatures, while echidnas - the anteaters - are terrestrial. Now Phillips and his colleagues suggest that platypuses and echidnas lived on because their ancestors sought refuge where marsupials could not follow - the water. The mystery then is why any monotremes survived. The struggle marsupials presumably had with all the animals on these continents during this journey might have primed them for competition, "while the Australian mammals that went extinct upon the arrival of marsupials had for the most part been isolated in Australia for a very long time," explained researcher Matthew Phillips, an evolutionary biologist at the Australian National University in Canberra.Īll these strengths help explain why marsupials triumphed in Australia. Moreover, before the marsupials reached Australia, they had migrated from Asia to the Americas to Antarctica. Marsupials appear to have a number of advantages over monotremes - their bodies seem more efficient at locomotion, and the fact that they give birth to live offspring could provide better care of young. The monotremes were almost totally swept aside when their pouch-bearing marsupial cousins - modern examples of which include the kangaroos - invaded Australia 71 million to 54 million years ago.
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